The present invention relates to a method for manufacturing a polymer coated long duration optical memory device having applications in ferroelectric liquid crystal materials.
Ferroelectric liquid crystal devices (FLCD) are widely used in spatial light modulators, light shutters, optical inter-connects, optical switches, optical gates for optical computing, high definition television terminals, electro-optic modulators etc. The optical addressed spatial light modulators has a great interest due to their applications in optical data processing, image amplification, incoherent to coherent image converters, wavelength converters, optical correlators etc. The surface bistability in ferroelectric liquid crystal materials in which the thickness of the cell is smaller than the pitch value of material, has a great potential from the application point of view. The fast switching bistable electro-optic devices based on ferroelectric liquid crystal material comprises a well aligned thin (1-3 xcexcm) layer of FLC sandwiched between two optically flat polished glass plates having transparent electrically conducting electrode patterns thereon. The two glass plates, are peripherically sealed and the electrical connections are taken out from the substrates. The display fixed with crossed polarizers on outer faces of both glass plates is connected with very large scale integrated (VLSI) electronic modules to be used as an information display for displaying electrical or optical data.
At present ferroelectric liquid crystal displays or devices are being prepared by
1. shearing the plates just below the smectic A-smectic C* (Sm C*xe2x88x92Sm A) transition temperature
2. by applying magnetic field
3. temperature gradient from spacer edge
4. depositing thin films of silicon monoxide on the supporting substrates with oblique angle of evaporation
5. polymer treated rubbed plates and
6. post treating the zig-zag defected sample with an ac voltage of 20 Vpp for few hours.
It is known in the state of the art that FLCD""s are being prepared by shearing the two glass plates at a temperature just below the Sm A-Sm C* phase transition. Smectic layers get aligned by applying shear and that layer structure is retained in Sm C* phase of liquid crystal to get an uniform defect free alignment of ferroelectric liquid crystals, reference may be made to N. A. Clark and S. T. Lagerwall, U.S. Pat. No. 4,563,059; N. A. Clark, and S. T. Lagerwall, Appl.Phys Lett, Vol.36, 899 (1980). The drawback of shearing technique is to get an uniform alignment over a large area because of the difficulty in applying uniform shear and to maintain a uniform temperature through out the display. The technique thus cannot be used as a commercially viable one.
It is also known in the state of art that the FLCD can be prepared by aligning the FLC molecules by magnetic field. Ferroelectric liquid crystal film encapsulated between transparent conducting glass plates is cooled slowly to Sm C* phase from its isotropic phase in strong uniform magnetic field. Reference may be made to A. M. Biradar, S. Wrobel and W. Haase, as described in Phys.Rev. A Vol. 39, 2693 (1989). The drawback in this process is to get uniform magnetic field over the large area and it takes long preparation time. Another known process in the state of the art is to grow smectic A phase under a temperature gradient as described by K. Ishikawa et. al; in Jpn. J. Appl. Phys. Vol.23, L211-213, (1984). The nucleation of the smectic A phase is initiated with the aid of spacer edge. The temperature gradient is obtained by using an Indium tin oxide (ITO) electrode properly etched as a local heater. This technique is very cumbersome and the maximum obtainable macrodomain is about one square millimeter in size.
Another process of making an uniform homogeneously aligned display is to deposit thin films of silicon monoxide at a pressure of 1xc3x9710xe2x88x925 torr, on the supporting transparent conducting glass plates with oblique angle of evaporation of 5xc2x0, 10xc2x0 and 15xc2x0 as described by Bawa et. al. in Appl. Phys. Lett. Vol.57, p.1398 (1990). The alignment of ferroelectric liquid crystal molecules is in the plane of evaporation but tilted with respect to the substrates depending upon the thickness of SiO2 to support the surface alignment. The display shows good bistability and is defect free. However, in this case the contrast is poor because of the high tilt angle of smectic layers (30xc2x0). Oblique deposition of SiO2 has also been studied for different angles of evoparation as described by Ouchi et. al. in Jpn. J.Appl.Phys, Vol.27, L725 (1988) and L. A. Goodman, in IEEE. Trans. Electron Devices ED-24, 995 (1977) and has been extensively used with nematic liquid crystals. Substrates deposited at 60xc2x0 angle with the normal, yield low (0xc2x0) pretilt surfaces as described by Y. Takanishi et.al; in Jpn. J. Appl. Phys; Vol. 28, L48 (1988).
Another known useful process in the state of the art is to align ferroelectric liquid crystal homogeneously, to rub unidirectionally the polyimide/polyamide treated transparent conducting plates as described by J. S. Patel et.al; in Ferroelectrics, Vol. 59, 137, (1984). Good alignment can be achieved by this technique over a large area. This method has been used to prepare the bistable devices as claimed by Yoshihara et al., in U.S. Pat. No. 5,568,299 and Tsuboyama et.al. in U.S. Pat. No. 05,013,137. However, the occurrence of zig-zag defects as described by M. A. Handschy and N. A. Clark; in Ferroelectrics, Vol. 59, 69 (1984) and line defects as described by Ishikawa et.al; in Jpn. J. Appl. Phys; Vol. 23, L 666 (1984) does not give rise to uniform contrast due to the presence of defect boundaries. Another drawback in this technique is that the charges are accumulated at the interface between the polymer and FLC material which destroys the bistability of the FLC device as described by Chieu and Yang, in Appl. Phys. Lett; Vol. 56, 1326 (1990).
Another known process in the state of art to get an uniform defect free sample is to post-treat the sample with AC field for a long time. The zig-zag type of defects which appears in a polyimide/polyamide treated rubbed glass plates can be removed by applying an alternating electric field of suitable frequency as detailed by S. S. Bawa et.al; in Jpn. J. Appl. Phys. Vol. 28, 662, (1989). The device can be obtained with large uniform area with good contrast and shows a bistability (memory effect) for few seconds. However, the long post-treatment timing makes the process to be a commercially non-viable one.
The main object of the present invention is to provide a method for the preparation of a polymer coated long duration optical memory device having applications in ferroelectric liquid crystal materials which obviates the drawbacks of hitherto known processes as described in Table-1.
Another object of the present invention is to provide a process for making an optical memory device having a high contrast display.
Still another object of the present invention is to provide an optical memory device having a microsecond switching time.
Yet another object of the invention is to provide a process for preparing an optical memory device using the homogeneous alignment of liquid crystal materials
Another object of the present invention is to provide a process for preparing a memory device by deposition of thick polymer coating on glass substrates.
Yet another object of the present invention is to provide an optical memory device having a threshold voltage of depolarizing field of 5xc3x97103 kV/cm.
Accordingly the present invention provides a method for the preparation of a polymer coated long duration optical memory device having applications in ferroelectric liquid crystal materials, the said method comprises:
(a) cleaning a pair of optically flat (xcex/10 flatness) glass plates;
(b) drying the cleaned glass plate;
(c) heating the dried glass plate to a temperature ranging between 100xc2x0 C.-250xc2x0 C. for a period ranging between 30 min. to 1 hr. in a vacuum chamber;
(d) coating the heated glass plates by depositing a thin film of indium tin oxide with a coating thickness in the range of 1000 xc3x85 to 2000 xc3x85 to obtain coated glass substrates with sheet resistance of at least 30-500 Qxcexa9/xe2x96xa1 and optical transmission of at least 85%;
e) cleaning the coated glass substrate obtained from step (d) above and forming patterns of different shapes and configurations by photolithographic methods to obtain an effective electrode area of at least 5 mm2;
(f) cleaning the coated glass substrate from step (e) above followed by deposition of antireflection coating of external surfaces of glass substrates;
(g) cleaning the coated glass substrate from step (f) above followed by coating of the patterned glass substrate with a polymer selected from the group consisting of polyamide consisting of nylon 6/6 and nylon 6/9 in the thickness range of 900-1100 xc3x85;
(h) baking the coated substrate obtained from the step (g) at a temperature ranging between 100xc2x0 C. and 120xc2x0 C. for a period ranging between 30 mins to 1 hr followed by hard rubbing of the polymer coated surface for number of rubbing steps in the range of 25 to 100, in the plane of the surface of the coated glass substrate;
(i) coating one of the substrates obtained from step (h) above with a spacer selected from photoresist, having a thickness in the range of 1 xcexcm to 3 xcexcm;
(j) inserting a ferroelectric liquid crystal material in the space available between the coated glass substrate from step (i) and glass substrate from step (h), followed by sealing the said sandwiched glass substrates at the periphery;
(k) heating the sandwiched glass substrates from step (j) above to a temperature in the range between 80xc2x0 C. to 100xc2x0 C. followed by cooling;
(l) fixing a polariser on non conducting surface of one of the glass substrates from step (k) above and an analyser on the non conducting surface of the other glass susbstrate;
(m) applying an AC field across device as obtained in step (l); and
(n) applying a DC field across the device for a time period in the range of 30 secs. to 2 minutes after performing step (m) to obtain an optical memory device having a long duration of memory of at least one year.
In an embodiment of the present invention the cleaning of the glass substrate in step (a) above may be accomplished by chemical methods and ultrasonic agitation.
In another embodiment of the present invention the cleaned glass substrates may be dried in inert gas selected from nitrogen, argon.
In yet another embodiment of the present invention the glass substrates used may be selected from the group consisting of scratch, stress and void free and may be optically polished to the flatness of at least xcex/4 per inch.
In still another embodiment the glass substrates used may be selected from the group consisting of fused Silica, Borosilicate material with low sodium ions and of 3-4 mm thickness to preserve the surface flatness.
In another embodiment the antireflection coatings may be multiple layers of SiO2 and TiO2 of a xcex/4 thickness.
In still another embodiment the transparent conducting coating used in step (g) above may be selected from indium tin oxide, tin oxide.
In yet another embodiment the antireflection coatings may be prepared by known methods of thin film deposition in step (d) may be done such as by sputtering, reactive evaporation, electron beam evaporation or by sol-gel method.
In another embodiment the transparent conducting films may be prepared by known methods of thin film deposition done in step (d) selected from sputtering, reactive evaporation and electron beam evaporation.
In still another embodiment the transparent conducting glass substrate as obtained in step (e) may be dipped in silane solution and then dried.
In an embodiment the glass substrates may be coated with polymer selected from the group consisting of polyamide and polyimide by using known spinner technique.
In yet another embodiment the spacer used on any of the glass substrate may be a film of photoresist prepared by known photolithographic technique.
In still another embodiment sealing of the periphery of the two glass substrates to prepare the display in step (k) may be carried out by thermal setting the thermoplastic, or UV sealant.
In yet another embodiment the polariser and analyser in step (m) may be fixed by adhesives.
In still another embodiment the AC electric field applied may be in the range of 2-20V peak to peak.
In yet another embodiment the AC field may be applied at a temperature below the transition temperature of smectic C* to chiral nematic phase.
In another embodiment the DC field may be applied for a time in the range of 30 secs to 2 minutes.
In another embodiment optical memory device is characterised by a threshold voltage of 5xc3x97103 kV/cm.
The expression glass substrates and glass substrates have been used interchangeably.
The glass plates of required size are initially edge polished. Preferably the glass plates are borosilicate, fused silica, or of quartz. Glass plates are then roughly grounded by known techniques to desired thickness by emery powders and then finally polished to achieve optical flatness of at least xcex/4 per inch using cerium oxide.
The glass plates are then cleaned to remove grease, dust etc. from their surfaces. Glass plates are initially boiled in chromic acid (K2Cr2O7+H2SO4+H2O) for 2-3 minutes and then cleaned in ultrasonic cleaner. Plates are then thoroughly cleaned sucessively in acetone, methanol and deionized water. Glass plates are then dried using filtered moisture free nitrogen gas.
The glass plates are then coated with a transparent conducting material under vacuum to form a thin layer of transparent conducting indium tin oxide, tin oxide, zinc oxide and the like. Glass plates are initially heated to 250xc2x0 C. in vacuum. Initial pressure attained in vacuum chamber is 10xe2x88x926 torr. Oxygen is introduced in the chamber to attain a pressure of about 2xc3x9710xe2x88x925 torr Indium oxide doped with about 3% metal tin is evaporated slowly by electron-beam gun to get deposited on hot glass plates. Oxygen in the chamber reacts with ongoing vapours to form a thin layer of indium tin oxide. The sheet resistance achieved in this case is in the range of 30 xcexa9/xe2x96xa1 to about 500 xcexa9/xe2x96xa1 with an optical transmission of more than 85% in the visible range.
The coated glass plates are cleaned again sequentially in soap solution, acetone, methanol and deionized water. The desired electrode pattern is formed by photolithographic and etching technique on the said glass plates. The glass substrates are spin coated initially with positive photoresist. Coated glass plates are prebaked at 80xc2x0 C. for 10 minutes. Plates are then exposed to UV radiations for 1-4 minutes with the negative of electrode pattern on the photoresist. The portion which is to be retained should be transparent to light in the pattern. The exposed plates are then developed using dilute (1:3) Kodak photoresist developer for one minute. The glass substrates are then post-baked to harden the patterned photoresist. Plates are then etched to remove undesired ITO conducting film. Photoresist from the glass plates is then wiped off using acetone.
The glass substrates, having desired transparent conducting pattern are initially coated with an adhesion promoter, commercially available VM 651 (Dupont) or a silane solution treatment. For silane (0.5% solution of phenyl trichloro silane in toluene) treatment the glass substrates are dipped for 10 minutes and are rinsed in propanol. This procedure is immediately followed by deposition of the polyamide nylon solution. The nylon solution is prepared by taking a 1.0% (wt. to vol.) of nylon 6/6 or nylon 6/9 and dissolved in 60% m-cresol and 40% methanol (vol. to vol.). A thin layer of this solution is applied to the glass substrates by spinning. In this case, the spinning ratio and the concentration determines the thickness of the polymer coating and is therefore very critical. First sufficient solution is kept on the glass substrate in order to cover the entire sample. The sample is then spun at 4000 RPM for not more than one minute. The excess solvent is allowed to evaporate by heating the glass substrates in an oven at a temperature not exceeding 130xc2x0 C. or a time period not more than one hour. Further, the polymer treated substrates are unidirectionally rubbed with a good quality velvet cloth. To achieve repeatability in the rubbed treatment, rubbing is done using a rubbing machine. In this machine, one can control the rubbing pressure as well as the number of rubbing strokes. The rubbing on the polymer coated glass substrates is obtained by means of a machine called screen printing machine. The distance between the rubbing block and the glass substrates is maintained by means of a head-screw. When the rubbing block just touches the glass substrates is termed as minimum rubbing and when head-screw completes one turn is taken as the maximum rubbing pressure.
The best bistable (memory) effect is observed when the thickness of the polymer coating is 900 to 1100 xc3x85 thick. When the thickness of the polymer is less than 900 xc3x85 the memory effect is lost or not observed. And if it is more than 1100 xc3x85 then the polymer coating is pealed off (removed) from the substrate at the time of rubbing, resulting into the disappearance of the alignment (non uniform contrast) in the displays. For achieving a good bistable (memory) device, based on FLC materials, a minimum distance (high rubbing pressure) is maintained between the rubbing block and the glass substrates. The main changes in the process steps as compared with the other known processes are firstly the thickness of the polymer coating is very thick (900 -1100 xc3x85) and secondly the rubbing strength on the polymer coated surface is relatively high. Because of these two conditions the surface area to accumulate the charges would be more.
Coating the polymer on one of the glass substrates and getting the uniform FLC alignment is not useful for all types of FLC materials. However, the contrast in one surface coated cells is very good (minimum defects). The Applicants have tested two FLC materials to get an alignment by treating one surface of the cell with polymer and rubbed.
After the alignment layer treatment by rubbing, the next step is to assemble the two glass plates to form a cell. The most important thing in this process is to obtain a uniform and accurate cell spacing. For this, on one of the glass substrates mylar spacers of known thicknesses are kept on the nonactive electrode area. The spacers are kept in such a way that there are two openings at the opposite sides to inject the liquid crystal material inside. Then the other glass plate is placed on to it so that the electrodes on them are properly matched to form an active display area. They are sealed together at the periphery, except the two openings, by Torr seal (Varian Associates, USA) or Araldite (CIBA, India).
However, achieving very thin samples (thickness of the order of 1-3 xcexcm) is very difficult by mylar spacers. Therefore, before the polymer treatment of the glass substrates, a thin layer of negative photoresist material is photolithographically formed on one of the substrate to form the spacer. By controlling the thickness of the photoresist layer one can control the thickness of the cell prepared. The photoresist coated glass plates are then baked at 200-250xc2x0 C. range for 30 minutes to make the photoresist coating hard. After this, the normal polymer coating and rubbing procedures are carried out and the cells are sealed. The cell thickness is measured by capacitance measurements or using optical interference method.
After the photoresist spacer and surface alignment coatings, the glass plates are assembled and sealed with a sealant and the ferroelectric liquid crystal material is filled in the cell. For this the liquid crystal material is placed near the small opening between the spacer and is filled by heating the sample to its isotropic phase. After the material has crept completely inside the cell, it is cooled slowly to ferroelectric liquid crystalline (Sm C*) phase. An electric field of 20 Vpp in frequency range of 50 Hz to 100 Hz is applied for one to two hours, just 1xc2x0 C. below the transition temperature of Sm C* to Sm A or Sm C* to N* phases, to get an excellent alignment of FLC molecules for homogeneous alignment (uniform contrast).
The main hypothesis in the present invention is to create more charge accumulation in between polymer layer and the FLC material interface by the application of an electric field in the surface stabilized geometry. Once the applied field is removed and if the field due to charge accumulation (depolarizing field) is more than the threshold field, the FLC molecules should switch in the reverse direction. When the FLC molecules switch in the reverse direction, there is no opposing force which can change the direction of the molecules and should remain in the reverse direction (memory state) for very long time. The field due to charge accumulation is enhanced by depositing thick polymer (900-1100 xc3x85) and strongly rubbing on the glass substrates, to provide more surface area for the accumulation of charges. In the cells where the thickness of the polymer is less than 900 xc3x85 and strongly rubbed the reverse memory effect is not observed, suggesting that the depolarizing field, created due to accumulation of charges is less than the threshold field, needed to switch the FLC molecules in reverse direction. If the thickness of the polymer is more than 1100 xc3x85 then the polymer coating is pealed off from the substrate at the time of rubbing.
In this invention to achieve the best charge stabilization (memory), a thick polymer (1000 xc3x85) and thin FLC layer (2 xcexcm) is better suited. During the application of an external field (+ve) the FLC molecules are in one of the uniform switched state (DOWN). The rotatory stage of the microscope was adjusted to achieve a dark state. After holding for 2 seconds-2 minutes (depending upon the amount of charges accumulated at the polymer surface) in this switched state, electric field is switched off and the field of view of the microscope turns bright. One can confirm that this state is indeed a reverse switched state (UP state) due to charge accumulation phenomenon, by the fact that when the sample is rotated by an angle 2xcex8 (≈45xc2x0) the field of view once again becomes dark. This charge stabilized UP state is highly stable and will remain in this state for months. This switched state acts like a monostable state but in reality it is not a true monostable state because of the fact that applying a field in the reverse direction (xe2x88x92ve) for another few seconds and switching off the field will bring back the sample to the DOWN state.
Thus, one can reliably obtain charge stabilized switched states by switching and holding the surface stabilized (SSFLC) structure in the opposite direction to the required state by applying an external field.
Filling hole is hermetically sealed with epoxy sealants. Analyser and polarizer are fixed on the two glass plates and the positions of the analyser and polarizer is adjusted such that they are crossed i.e. position of the polarizer is rotated by 90xc2x0 with respect to the analyser. Electrical connections are taken out from the conducting portion of the electrodes for applying the electric field. The memory characteristics of the device of present invention are described in table-2